ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I a) there is a net force but the book has too much inertia b) there are no forces.

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Presentation on theme: "ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I a) there is a net force but the book has too much inertia b) there are no forces."— Presentation transcript:

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ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I a) there is a net force but the book has too much inertia b) there are no forces acting on it at all c) it does move, but too slowly to be seen d) there is no net force on the book e) there is a net force, but the book is too heavy to move A book is lying at rest on a table. The book will remain there at rest because:

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There are forces acting on the book leaving no net force There are forces acting on the book, but the only forces acting are in the y-direction. Gravity acts downward, but the table exerts an upward force that is equally strong, so the two forces cancel, leaving no net force. ConcepTest 4.1aNewton’s First Law I ConcepTest 4.1a Newton’s First Law I a) there is a net force but the book has too much inertia b) there are no forces acting on it at all c) it does move, but too slowly to be seen d) there is no net force on the book e) there is a net force, but the book is too heavy to move A book is lying at rest on a table. The book will remain there at rest because:

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ConcepTest 4.1bNewton’s First Law II ConcepTest 4.1b Newton’s First Law II a) more than its weight b) equal to its weight c) less than its weight but more than zero d) depends on the speed of the puck e) zero A hockey puck slides on ice at constant velocity. What is the net force acting on the puck?

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constant velocity not accelerating no net force The puck is moving at a constant velocity, and therefore it is not accelerating. Thus, there must be no net force acting on the puck. ConcepTest 4.1bNewton’s First Law II ConcepTest 4.1b Newton’s First Law II a) more than its weight b) equal to its weight c) less than its weight but more than zero d) depends on the speed of the puck e) zero A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? Follow-up: Are there any forces acting on the puck? What are they?

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a) a net force acted on it b) no net force acted on it c) it remained at rest d) it did not move, but only seemed to e) gravity briefly stopped acting on it ConcepTest 4.1cNewton’s First Law III ConcepTest 4.1c Newton’s First Law III You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why?

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a) a net force acted on it b) no net force acted on it c) it remained at rest d) it did not move, but only seemed to e) gravity briefly stopped acting on it continued moving forwardinitial state of motion The book was initially moving forward (since it was on a moving bus). When the bus stopped, the book continued moving forward, which was its initial state of motion, and therefore it slid forward off the seat. ConcepTest 4.1cNewton’s First Law III ConcepTest 4.1c Newton’s First Law III You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why? Follow-up: What is the force that usually keeps the book on the seat?

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ConcepTest 4.1dNewton’s First Law IV ConcepTest 4.1d Newton’s First Law IV a) the force pushing the stone forward finally stopped pushing on it b) no net force acted on the stone c) a net force acted on it all along d) the stone simply “ran out of steam” e) the stone has a natural tendency to be at rest You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that:

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some forceto slow it down and stop it After the stone was kicked, no force was pushing it along! However, there must have been some force acting on the stone to slow it down and stop it. This would be friction!! ConcepTest 4.1dNewton’s First Law IV ConcepTest 4.1d Newton’s First Law IV a) the force pushing the stone forward finally stopped pushing on it b) no net force acted on the stone c) a net force acted on it all along d) the stone simply “ran out of steam” e) the stone has a natural tendency to be at rest You kick a smooth flat stone out on a frozen pond. The stone slides, slows down and eventually stops. You conclude that:

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ConcepTest 4.2aCart on Track I ConcepTest 4.2a Cart on Track I a) slowly come to a stop b) continue with constant acceleration c) continue with decreasing acceleration d) continue with constant velocity e) immediately come to a stop Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart?

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ConcepTest 4.2aCart on Track I ConcepTest 4.2a Cart on Track I a) slowly come to a stop b) continue with constant acceleration c) continue with decreasing acceleration d) continue with constant velocity e) immediately come to a stop Consider a cart on a horizontal frictionless table. Once the cart has been given a push and released, what will happen to the cart? no longer a force This does not mean that the cart stops moving!!continue moving with the same velocity After the cart is released, there is no longer a force in the x-direction. This does not mean that the cart stops moving!! It simply means that the cart will continue moving with the same velocity it had at the moment of release. The initial push got the cart moving, but that force is not needed to keep the cart in motion.

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ConcepTest 4.2bCart on Track II ConcepTest 4.2b Cart on Track II We just decided that the cart continues with constant velocity. What would have to be done in order to have the cart continue with constant acceleration? a) push the cart harder before release b) push the cart longer before release c) push the cart continuously d) change the mass of the cart e) it is impossible to do that

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maintain the applied force continue pushing In order to achieve a non-zero acceleration, it is necessary to maintain the applied force. The only way to do this would be to continue pushing the cart as it moves down the track. This will lead us to a discussion of Newton’s Second Law. ConcepTest 4.2bCart on Track II ConcepTest 4.2b Cart on Track II We just decided that the cart continues with constant velocity. What would have to be done in order to have the cart continue with constant acceleration? a) push the cart harder before release b) push the cart longer before release c) push the cart continuously d) change the mass of the cart e) it is impossible to do that

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ConcepTest 4.3Truck on Frozen Lake ConcepTest 4.3 Truck on Frozen Lake A very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck? a) it is too heavy, so it just sits there b) it moves backward at const. speed c) it accelerates backward d) it moves forward at const. speed e) it accelerates forward

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When the fly hit the truck, it exerted a force on the truck (only for a fraction of a second). So, in this time period, the truck accelerated (backwards) up to some speed. After the fly was squashed, it no longer exerted a force, and the truck simply continued moving at constant speed. ConcepTest 4.3Truck on Frozen Lake ConcepTest 4.3 Truck on Frozen Lake A very large truck sits on a frozen lake. Assume there is no friction between the tires and the ice. A fly suddenly smashes against the front window. What will happen to the truck? a) it is too heavy, so it just sits there b) it moves backward at const. speed c) it accelerates backward d) it moves forward at const. speed e) it accelerates forward

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ConcepTest 4.4aOff to the Races I ConcepTest 4.4a Off to the Races I a) 16 s b) 8 s c) 4 s d) 2 s e) 1 s From rest, we step on the gas of our Ferrari, providing a force F for 4 secs, speeding it up to a final speed v. If the applied force were only 1/2 F, how long would it have to be applied to reach the same final speed? v F

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T = 4 s v = aT half half time must be doubled In the first case, the acceleration acts over time T = 4 s to give velocity v = aT. In the second case, the force is half, therefore the acceleration is also half, so to achieve the same final speed, the time must be doubled. ConcepTest 4.4aOff to the Races I ConcepTest 4.4a Off to the Races I a) 16 s b) 8 s c) 4 s d) 2 s e) 1 s From rest, we step on the gas of our Ferrari, providing a force F for 4 secs, speeding it up to a final speed v. If the applied force were only 1/2 F, how long would it have to be applied to reach the same final speed? v F

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ConcepTest 4.5Force and Mass ConcepTest 4.5 Force and Mass a) 4 v b) 2 v c) v d) 1/2 v e) 1/4 v A force F acts on mass M for a time interval T, giving it a final speed v. If the same force acts for the same time on a different mass 2M, what would be the final speed of the bigger mass?

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T v = aTdoubled half Tfinal speed will only be half In the first case, the acceleration acts over time T to give velocity v = aT. In the second case, the mass is doubled, so the acceleration is cut in half, therefore, in the same time T, the final speed will only be half as much. ConcepTest 4.5Force and Mass ConcepTest 4.5 Force and Mass a) 4 v b) 2 v c) v d) 1/2 v e) 1/4 v A force F acts on mass M for a time interval T, giving it a final speed v. If the same force acts for the same time on a different mass 2M, what would be the final speed of the bigger mass?

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F a1a1 m1m1 F m2m2 m1m1 a3a3 a) 3/4 a 1 b) 3/2 a 1 c) 1/2 a 1 d) 4/3 a 1 e) 2/3 a 1 A force F acts on mass m 1 giving acceleration a 1. The same force acts on a different mass m 2 giving acceleration a 2 = 2a 1. If m 1 and m 2 are glued together and the same force F acts on this combination, what is the resulting acceleration? F a 2 = 2a 1 m2m2 ConcepTest 4.6Force and Two Masses ConcepTest 4.6 Force and Two Masses

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m 2 (1/2)m 1 2a 1 (3/2)m 1 (2/c)a 1 same applied force. Mass m 2 must be (1/2)m 1 because its acceleration was 2a 1 with the same force. Adding the two masses together gives (3/2)m 1, leading to an acceleration of (2/c)a 1 for the same applied force. F = m 1 a 1 F a1a1 m1m1 F m2m2 m1m1 a3a3 F = (3/2)m 1 a 3 => a 3 = (2/3) a 1 a) 3/4 a 1 b) 3/2 a 1 c) 1/2 a 1 d) 4/3 a 1 e) 2/3 a 1 A force F acts on mass m 1 giving acceleration a 1. The same force acts on a different mass m 2 giving acceleration a 2 = 2a 1. If m 1 and m 2 are glued together and the same force F acts on this combination, what is the resulting acceleration? F a 2 = 2a 1 m2m2 F = m 2 a 2 = (1/2 m 1 )(2a 1 ) ConcepTest 4.6Force and Two Masses ConcepTest 4.6 Force and Two Masses

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ConcepTest 4.7aGravity and Weight I ConcepTest 4.7a Gravity and Weight I a) F g is greater on the feather b) F g is greater on the stone c) F g is zero on both due to vacuum d) F g is equal on both always e) F g is zero on both always What can you say about the force of gravity F g acting on a stone and a feather?

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The force of gravity (weight) depends on the mass of the object!! The stone has more mass, therefore more weight. ConcepTest 4.7aGravity and Weight I ConcepTest 4.7a Gravity and Weight I a) F g is greater on the feather b) F g is greater on the stone c) F g is zero on both due to vacuum d) F g is equal on both always e) F g is zero on both always What can you say about the force of gravity F g acting on a stone and a feather?

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a) it is greater on the feather b) it is greater on the stone c) it is zero on both due to vacuum d) it is equal on both always e) it is zero on both always What can you say about the acceleration of gravity acting on the stone and the feather? ConcepTest 4.7bGravity and Weight II ConcepTest 4.7b Gravity and Weight II

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F/m mg g The acceleration is given by F/m so here the mass divides out. Since we know that the force of gravity (weight) is mg, then we end up with acceleration g for both objects. a) it is greater on the feather b) it is greater on the stone c) it is zero on both due to vacuum d) it is equal on both always e) it is zero on both always What can you say about the acceleration of gravity acting on the stone and the feather? ConcepTest 4.7bGravity and Weight II ConcepTest 4.7b Gravity and Weight II Follow-up: Which one hits the bottom first?

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ConcepTest 4.9aGoing Up I ConcepTest 4.9a Going Up I A block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block? a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 e) depends on the size of the elevator m v

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no net force NmgNmg The block is moving at constant speed, so it must have no net force on it. The forces on it are N (up) and mg (down), so N = mg, just like the block at rest on a table. ConcepTest 4.9aGoing Up I ConcepTest 4.9a Going Up I A block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block? a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 e) depends on the size of the elevator m v

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A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block? a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 e) depends on the size of the elevator ConcepTest 4.9bGoing Up II ConcepTest 4.9b Going Up II m a

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net upward force Nmg Nmg net upward force The block is accelerating upward, so it must have a net upward force. The forces on it are N (up) and mg (down), so N must be greater than mg in order to give the net upward force! a) N > mg b) N = mg c) N < mg (but not zero) d) N = 0 e) depends on the size of the elevator  F = N – mg = ma > 0  N > mg m a > 0 mg N A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block? ConcepTest 4.9bGoing Up II ConcepTest 4.9b Going Up II

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ConcepTest 4.10Normal Force ConcepTest 4.10 Normal Force Case 1 Case 2 Below you see two cases: a physics student pulling or pushing a sled with a force F which is applied at an angle . In which case is the normal force greater? a) case 1 b) case 2 c) it’s the same for both d) depends on the magnitude of the force F e) depends on the ice surface

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down in addition to mg larger up lessened In Case 1, the force F is pushing down (in addition to mg), so the normal force needs to be larger. In Case 2, the force F is pulling up, against gravity, so the normal force is lessened. ConcepTest 4.10Normal Force ConcepTest 4.10 Normal Force Case 1 Case 2 Below you see two cases: a physics student pulling or pushing a sled with a force F which is applied at an angle . In which case is the normal force greater? a) case 1 b) case 2 c) it’s the same for both d) depends on the magnitude of the force F e) depends on the ice surface

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ConcepTest 4.11On an Incline ConcepTest 4.11 On an Incline a) case A b) case B c) both the same (N = mg) d) both the same (0 < N < mg) e) both the same (N = 0) Consider two identical blocks, one resting on a flat surface, and the other resting on an incline. For which case is the normal force greater?

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a) case A b) case B c) both the same (N = mg) d) both the same (0 < N < mg) e) both the same (N = 0) N W WyWy x y f   ConcepTest 4.11On an Incline ConcepTest 4.11 On an Incline Consider two identical blocks, one resting on a flat surface, and the other resting on an incline. For which case is the normal force greater? Case AN = W Case B N < W In Case A, we know that N = W. In Case B, due to the angle of the incline, N < W. In fact, we can see that N = W cos(  ).

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F F 12 F F 21 a) a) the bowling ball exerts a greater force on the ping-pong ball b) b) the ping-pong ball exerts a greater force on the bowling ball c) c) the forces are equal d) d) the forces are zero because they cancel out e) there are actually no forces at all ConcepTest 4.13aBowling vs. Ping-Pong I ConcepTest 4.13a Bowling vs. Ping-Pong I In outer space, a bowling ball and a ping-pong ball attract each other due to gravitational forces. How do the magnitudes of these attractive forces compare?

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F F 12 F F 21 forces The forces are equal and opposite by Newton’s 3 rd Law! a) a) the bowling ball exerts a greater force on the ping-pong ball b) b) the ping-pong ball exerts a greater force on the bowling ball c) c) the forces are equal d) d) the forces are zero because they cancel out e) there are actually no forces at all ConcepTest 4.13aBowling vs. Ping-Pong I ConcepTest 4.13a Bowling vs. Ping-Pong I In outer space, a bowling ball and a ping-pong ball attract each other due to gravitational forces. How do the magnitudes of these attractive forces compare?

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In outer space, gravitational forces exerted by a bowling ball and a ping-pong ball on each other are equal and opposite. How do their accelerations compare? a) a) they do not accelerate because they are weightless b) b) accels. are equal, but not opposite c) c) accelerations are opposite, but bigger for the bowling ball d) d) accelerations are opposite, but bigger for the ping-pong ball e) accels. are equal and opposite ConcepTest 4.13bBowling vs. Ping-Pong II ConcepTest 4.13b Bowling vs. Ping-Pong II F F 12 F F 21

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forces smaller massbigger acceleration The forces are equal and opposite -- this is Newton’s 3 rd Law!! But the acceleration is F/m and so the smaller mass has the bigger acceleration. In outer space, gravitational forces exerted by a bowling ball and a ping-pong ball on each other are equal and opposite. How do their accelerations compare? a) a) they do not accelerate because they are weightless b) b) accels. are equal, but not opposite c) c) accelerations are opposite, but bigger for the bowling ball d) d) accelerations are opposite, but bigger for the ping-pong ball e) accels. are equal and opposite ConcepTest 4.13bBowling vs. Ping-Pong II ConcepTest 4.13b Bowling vs. Ping-Pong II F F 12 F F 21

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ConcepTest 4.16aTension I ConcepTest 4.16a Tension I a) 0 N b) 50 N c) 100 N d) 150 N e) 200 N You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope?

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The tension in the rope is the force that the rope “feels” across any section of it (or that you would feel if you replaced a piece of the rope). Since you are pulling with a force of 100 N, that is the tension in the rope. ConcepTest 4.16aTension I ConcepTest 4.16a Tension I a) 0 N b) 50 N c) 100 N d) 150 N e) 200 N You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope?

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literally The tension is not 200 N !! 100 N This is literally the identical situation to the previous question. The tension is not 200 N !! Whether the other end of the rope is pulled by a person, or pulled by a tree, the tension in the rope is still 100 N !! a) 0 N b) 50 N c) 100 N d) 150 N e) 200 N Two tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope? ConcepTest 4.16bTension II ConcepTest 4.16b Tension II

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a) you and your friend each pull on opposite ends of the rope b) tie the rope to a tree, and you both pull from the same end c) it doesn’t matter -- both of the above are equivalent d) get a large dog to bite the rope You and a friend can each pull with a force of 20 N. If you want to rip a rope in half, what is the best way? ConcepTest 4.16cTension III ConcepTest 4.16c Tension III

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Take advantage of the fact that the tree can pull with almost any force (until it falls down, that is!). You and your friend should team up on one end, and let the tree make the effort on the other end. a) you and your friend each pull on opposite ends of the rope b) tie the rope to a tree, and you both pull from the same end c) it doesn’t matter -- both of the above are equivalent d) get a large dog to bite the rope You and a friend can each pull with a force of 20 N. If you want to rip a rope in half, what is the best way? ConcepTest 4.16cTension III ConcepTest 4.16c Tension III

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ConcepTest 4.19Friction ConcepTest 4.19 Friction a) the force from the rushing air pushed it off b) the force of friction pushed it off c) no net force acted on the box d) truck went into reverse by accident e) none of the above A box sits in a pickup truck on a frictionless truck bed. When the truck accelerates forward, the box slides off the back of the truck because:

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friction If there is no friction, there is no force to push the box along, and it remains at rest. Generally, the reason that the box in the truck bed would move with the truck is due to friction between the box and the bed. If there is no friction, there is no force to push the box along, and it remains at rest. The truck accelerated away, essentially leaving the box behind!! ConcepTest 4.19Friction ConcepTest 4.19 Friction a) the force from the rushing air pushed it off b) the force of friction pushed it off c) no net force acted on the box d) truck went into reverse by accident e) none of the above A box sits in a pickup truck on a frictionless truck bed. When the truck accelerates forward, the box slides off the back of the truck because:

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Antilock brakes keep the car wheels from locking and skidding during a sudden stop. Why does this help slow the car down? a)  k >  s so sliding friction is better b)  k >  s so static friction is better c)  s >  k so sliding friction is better d)  s >  k so static friction is better e) none of the above Static friction is greater than sliding friction Static friction is greater than sliding friction, so by keeping the wheels from skidding, the static friction force will help slow the car down more efficiently than the sliding friction that occurs during a skid. ConcepTest 4.20Antilock Brakes ConcepTest 4.20 Antilock Brakes

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ConcepTest 4.21Going Sledding ConcepTest 4.21 Going Sledding 1 2 a) pushing her from behind b) pulling her from the front c) both are equivalent d) it is impossible to move the sled e) tell her to get out and walk Your little sister wants you to give her a ride on her sled. On level ground, what is the easiest way to accomplish this?

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ConcepTest 4.21Going Sledding ConcepTest 4.21 Going Sledding 1 2 pushing down normal force is larger pulling up normal force is lessened In Case 1, the force F is pushing down (in addition to mg), so the normal force is larger. In Case 2, the force F is pulling up, against gravity, so the normal force is lessened. Recall that the frictional force is proportional to the normal force. a) pushing her from behind b) pulling her from the front c) both are equivalent d) it is impossible to move the sled e) tell her to get out and walk Your little sister wants you to give her a ride on her sled. On level ground, what is the easiest way to accomplish this?

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ConcepTest 4.22Will It Budge? ConcepTest 4.22 Will It Budge? a) moves to the left b) moves to the right c) moves up d) moves down e) the box does not move A box of weight 100 N is at rest on a floor where  s = 0.5. A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move? T m Static friction (  s  = 0.4  )

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maximum  s N = 40 N 30 N The static friction force has a maximum of  s N = 40 N. The tension in the rope is only 30 N. So the pulling force is not big enough to overcome friction. ConcepTest 4.22Will It Budge? ConcepTest 4.22 Will It Budge? a) moves to the left b) moves to the right c) moves up d) moves down e) the box does not move A box of weight 100 N is at rest on a floor where  s = 0.5. A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move? T m Static friction (  s  = 0.4  )

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a) component of the gravity force parallel to the plane increased b) coeff. of static friction decreased c) normal force exerted by the board decreased d) both #1 and #3 e) all of #1, #2, and #3 A box sits on a flat board. You lift one end of the board, making an angle with the floor. As you increase the angle, the box will eventually begin to slide down. Why? Net Force Normal Weight ConcepTest 4.23aSliding Down I ConcepTest 4.23a Sliding Down I

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a) component of the gravity force parallel to the plane increased b) coeff. of static friction decreased c) normal force exerted by the board decreased d) both #1 and #3 e) all of #1, #2, and #3 A box sits on a flat board. You lift one end of the board, making an angle with the floor. As you increase the angle, the box will eventually begin to slide down. Why? Net Force Normal Weight component of weight parallel to the plane increases component perpendicular to the plane decreases friction force gets smallerforce pulling the box down the plane gets bigger l As the angle increases, the component of weight parallel to the plane increases and the component perpendicular to the plane decreases (and so does the Normal force). Since friction depends on Normal force, we see that the friction force gets smaller and the force pulling the box down the plane gets bigger. ConcepTest 4.23aSliding Down I ConcepTest 4.23a Sliding Down I